Process for producing articles formed from polylactic acid and articles made therefrom

ABSTRACT

PLA polymers that can be expanded into microporous articles having a node and fibril microstructure are provided. The fibrils contain PLA polymer chains oriented with the fibril axis. Additionally, the PLA polymers have an inherent viscosity greater than about 3.8 dL/g and a calculated molecular weight greater than about 150,000 g/mol. The PLA polymer article may be formed by bulk polymerization where the PLA bulk polymer is made into a preform that is subsequently expanded at temperatures above the glass transition temperature and below the melting point of the PLA polymer. In an alternate embodiment, a PLA polymer powder is lubricated, the lubricated polymer is subjected to pressure and compression to form a preform, and the preform is expanded to form a microporous article. Both the preform and the microporous article are formed at temperatures above the glass transition temperature and below the melting point of the PLA polymer.

FIELD

The present invention relates generally to polylactic acid (PLA), andmore specifically to articles formed from polylactic acid that arebiodegradable, possess a high modulus of elasticity, and increasedtensile strength. Processes for forming both dense and porous articlesfrom polylactic acid are also provided.

BACKGROUND

Polylactic acids (PLA) are aliphatic polyesters and are considereduseful biodegradable materials because of their hydrolytic property.Additionally, the degradative product of polylactic acid (i.e., lacticacid) is readily absorbed in vivo. As such, PLA is commonly used formedical purposes, such as in surgical sutures, in sustained-releasecapsules in drug delivery systems, and as reinforcing materials for bonefractures. However, conventional processes for forming PLA articlespossess undesirable features or attributes, such as, for example,processing above the melt temperature of the PLA polymer that degradesthe PLA polymers and undesirably colors the polymer, and reducesphysical properties such as tensile strength and matrix modulus.Additionally, conventional process are forced to use low molecularweight PLA due to the high melt viscosity.

Thus, there exists a need in the art for a process for making a PLApolymer and a PLA polymer article that is biodegradable, possesses ahigh modulus of elasticity, and has increased tensile strength.

SUMMARY

One embodiment relates to an article that includes an expanded PLApolymer having a beta crystal phase and nodes and fibrils. The fibrilsinclude polymer chains oriented along a fibril axis. In at least oneembodiment, the PLA polymer includes at least one comonomer.Additionally, the PLA polymer has a first melt enthalpy greater thanabout 30 J/g, an inherent viscosity greater than about 3.8 dL/g and acalculated molecular weight greater than about 190,000 g/mol. The PLApolymer article may have a matrix tensile strength greater than or equalto 110 MPa, or 115 MPa, and a matrix modulus greater than or equal to3000 MPa. A filler material and/or a coating material may be placed onand/or in the PLA article. In one or more embodiment, the PLA article ismicroporous. The article may be densified to form a dense article havinga porosity less than about 10%.

A second embodiment relates to a process for forming a porous articlethat includes expanding a PLA polymer preform at a temperature above theglass transition temperature of the PLA polymer and below a meltingtemperature of the PLA polymer to create a porous PLA article havingnodes and fibrils. The preform may be in the form of film, rods, tubes,or discs. In at least one embodiment, the expansion of the preformoccurs at a temperature from about 60° C. to about 185° C. A fillermaterial and/or a coating material may be placed on and/or in the PLAarticle. In one or more embodiment, the porous PLA article ismicroporous. Additionally, the porous article may be compressed at atemperature below the melting temperature of the PLA polymer to form adense PLA article that has a porosity less than about 10%.

A third embodiment relates to a process for forming a microporousarticle that includes (1) lubricating a PLA polymer powder to form alubricated PLA polymer, (2) subjecting the lubricated PLA polymer topressure and to a temperature above the glass transition temperature ofthe PLA polymer and below a melting temperature of the PLA polymer toform a preform, and (3) expanding the preform at a temperature below themelt temperature of the PLA polymer to form a porous article having astructure of nodes and fibrils. The porous article may be a microporousarticle. In at least one embodiment, the lubricated PLA polymer may becalendered or ram extruded below the melt temperature of the PLApolymer. In one or more embodiment, the PLA polymer is expanded at atemperature that is about 80° C. or less below the melt temperature. Thelubricant may be removed from the preform prior to expanding thepreform. In a further embodiment, the porous article may be compressedto form a dense article having a porosity of less than about 10%.

A fourth embodiment relates to a process for forming a porous articlethat includes (1) lubricating a PLA polymer powder to form a lubricatedPLA polymer and (2) calendering the lubricated PLA polymer at atemperature above the glass transition temperature of said PLA polymerand below a melting temperature of the PLA polymer to form a porousarticle having a structure of nodes and fibrils. In at least oneembodiment, the calendering occurs at a temperature from about 60° C. toabout 185° C. In a further embodiment, the lubricant is removed from theporous article to form a non-lubricated article and the non-lubricatedarticle is calendered to form a dense article having a porosity lessthan about 10%.

A fifth embodiment relates to a process for forming a dense article thatincludes applying pressure and heat (e.g. calendering) to a PLA polymerpowder having an inherent viscosity greater than about 3.8 dL/g and amolecular weight greater than about 190,000 g/mol at a temperature belowthe melt temperature of the PLA polymer to form a dense article (e.g., acalendered PLLA film). The dense article may be drawn in one or moredirection at a temperature below the melt temperature of the PLA polymerto form a second dense article (e.g., fibrillated (dense) article). Thefibrillated (dense) article has a structure of nodes and fibrils and abeta crystal phase. The dense article has a porosity less than about10%.

A sixth embodiment relates to a dense article that includes a PLA filmhaving a porosity of less than 10%.

A seventh embodiment relates to an article that includes (1) an expandedPLA polymer comprising a beta crystal phase and having nodes and fibrilsand (2) at least one filler material. The filler material may includeinorganic materials (e.g., silica) carbon black, aerogels, metals,semi-metals, ceramics, carbon/metal particulate blends, activatedcarbon, hydrogel materials, bioactive substances, stiffening agents, andcombinations thereof.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings are included to provide a furtherunderstanding of the disclosure and are incorporated in and constitute apart of this specification, illustrate embodiments, and together withthe description serve to explain the principles of the disclosure.

FIG. 1 is a wide angle X-ray diffraction (WAXD) pattern of the PLLApolymer of Example 2 according to at least one embodiment;

FIG. 2 is a wide angle X-ray diffraction (WAXD) pattern of the PLLApolymer of Example 3 in accordance with at least one embodiment;

FIG. 3 is an intensity vs. 2-theta plot of the wide angle X-raydiffraction (WAXD) patterns of FIG. 1 and FIG. 2 comparing theintegrated intensity of Example 2 with the meridonal intensity ofExample 3 in accordance with at least one embodiment;

FIG. 4 is a differential scanning calorimetry (DSC) thermogram depictinga peak melting temperature and melt enthalpy of the unexpanded PLLApolymer of Example 1 according to at least one embodiment;

FIG. 5 is a differential scanning calorimetry (DSC) thermogram depictinga peak melting temperature and melt enthalpy of the expanded porous PLLApolymer of Example 3 according to at least one embodiment;

FIG. 6 is a differential scanning calorimetry (DSC) thermogram depictinga peak melting temperature and melt enthalpy of the expanded porous PLLApolymer of Example 4 in accordance with one embodiment;

FIG. 7 is a differential scanning calorimetry (DSC) thermogram depictinga peak melting temperature and melt enthalpy of the expanded porous PLLApolymer of Example 5 according to at least one embodiment;

FIG. 8 is a scanning electron micrograph (SEM) of the surface of aninterior view of the expanded porous PLLA polymer of Example 3 taken at40,000× magnification in accordance with one embodiment of theinvention;

FIG. 9 is a scanning electron micrograph (SEM) of the surface of aninterior view of the expanded porous PLLA polymer of Example 4 taken at20,000× magnification according to an embodiment;

FIG. 10 is a scanning electron micrograph (SEM) of the surface of aninterior view of the expanded porous PLLA polymer of Example 5 taken at35,000× magnification in accordance with an embodiment;

FIG. 11 is a scanning electron micrograph (SEM) of the surface of aninterior view of the expanded porous PLLA polymer of Example 14 taken at8,000× magnification in accordance with an embodiment;

FIG. 12 is a scanning electron micrograph (SEM) of the surface of aninterior view of the expanded porous PLLA polymer of Example 15 taken at45,000× magnification according to one embodiment;

FIG. 13 is a graphical illustration of the data from Table 2 showing theformation of a microporous structure as a function of melt enthalpy andinherent viscosity according to one embodiment;

FIG. 14 is a scanning electron micrograph (SEM) of the surface of aninterior view of the continuous, cohesive opaque tape of Example 30taken at 35,000× magnification in accordance with an embodiment;

FIG. 15 is a scanning electron micrograph (SEM) of the cross-section ofthe tape produced in Example 29 taken at 15,000× magnification accordingto one embodiment;

FIG. 16 is a scanning electron micrograph (SEM) of the surface of thesheet produced in Example 27 taken at 20,000× magnification inaccordance with one embodiment;

FIG. 17 is a powder X-ray diffraction (XRD) pattern plotting intensityvs. 2-theta indicating the formation of a beta crystal phase in acalendered PLA article of Example 29 in accordance with one embodiment;

FIG. 18 is a wide angle X-ray diffraction (WAXD) pattern of the PLLApowder of Example 34 in accordance with at least one embodiment;

FIG. 19 is an intensity vs. 2-theta plot of the wide angle X-raydiffraction (WAXD) patterns of FIG. 18 according to at least oneembodiment;

FIG. 20 is a scanning electron micrograph (SEM) of the surface of thePLLA film of Example 35 taken at 35,000× magnification in accordancewith an embodiment;

FIG. 21 is a scanning electron micrograph (SEM) of the surface of thePLLA film of Example 36 taken at 45,000× magnification in accordancewith an embodiment;

FIG. 22 is a wide angle x-ray diffraction (WAXD) pattern of the PLLAfilm of Example 36 according to one embodiment;

FIG. 23 is an intensity vs. 2-theta plot of the wide angle X-raydiffraction (WAXD) patterns of FIG. 22 in accordance with an embodiment;

FIG. 24 is a scanning electron micrograph (SEM) of the surface of thePLLA film of Example 37 taken at 25,000× magnification in accordancewith an embodiment;

FIG. 25 is a scanning electron micrograph (SEM) of the surface of thePLLA film of Example 39 taken at 25,000× magnification in accordancewith an embodiment;

FIG. 26 is a wide angle x-ray diffraction (WAXD) pattern of the PLLAfilm of Example 39 in accordance with one exemplary embodiment;

FIG. 27 is an intensity vs. 2-theta plot of the wide angle X-raydiffraction (WAXD) pattern of FIG. 26 according to at least oneembodiment;

FIG. 28 is a scanning electron micrograph (SEM) of the surface of thecompressed PLLA polymer of Example 32 taken at 30,000× magnificationaccording to at least one exemplary embodiment; and

FIG. 29 is a scanning electron micrograph (SEM) of the PLLA/silica filmof Example 42 taken at 25,000× according to an exemplary embodiment.

GLOSSARY

As used herein, the term “lubricant” is meant to describe a processingaid that includes, and in some embodiments, consists of, anincompressible fluid that is not a solvent for the polymer at processingconditions. The fluid-polymer surface interactions are such that it ispossible to create a homogenous mixture.

As used herein, the term “PLA” refers to polylactic acid andpolylactide.

As used herein, the term “PLLA” refers to poly L-lactic acid and polyL-lactide.

As used herein, the term “PDLA” refers to poly D-lactic acid and polyD-lactide.

As used herein, the term “cohesive” is meant to describe a tape orprecursor material that is sufficiently strong for further processing.

As used herein, the term “uniaxial” is meant to describe a polymer,membrane, preform, or article that is expanded in only one direction.

As used herein, the term “biaxial” is meant to describe a polymer,membrane, preform, or article that is expanded in at least twodirections.

As used herein, the term “microporous” is meant to define an article,such as a membrane, that has pores that are not visible to the nakedeye.

As used herein, the terms “melting point”, “melt temperature”, and“melting temperature” are used interchangeably herein and are meant todefine the peak of the melt endotherm as measured by differentialscanning calorimetry (DSC) during the first heating of the PLA polymer.

As used herein, the term “fibril axis” is meant to describe the longdimension of the fibril.

As used herein, the terms “dense” and “densified” are meant to describea PLA polymer article that has a porosity less than about 10%.

DETAILED DESCRIPTION

Persons skilled in the art will readily appreciate that various aspectsof the present disclosure can be realized by any number of methods andapparatus configured to perform the intended functions. It should alsobe noted that the accompanying figures referred to herein are notnecessarily drawn to scale, but may be exaggerated to illustrate variousaspects of the present disclosure, and in that regard, the figuresshould not be construed as limiting.

The present disclosure relates to polylactic acid (PLA) polymers andporous articles made therefrom that are bioabsorbable, biodegradable,and possess a high modulus of elasticity and increased tensile strength.The crystallinity of the PLA polymer may be measured by differentialscanning calorimetry (DSC). The melt enthalpy of the PLA polymer asmeasured by DSC is 30 J/g or greater, and in some embodiments, 40 J/g orgreater. The PLA polymers can be formed into microporous articles attemperatures below the melting point of the PLA polymer. The PLA polymerarticles have a node and fibril microstructure. In at least oneembodiment, the fibrils contain PLA polymer chains oriented with thefibril axis. With reference to the PLA polymers and PLA articlesdescribed herein, a node may be described as a larger volume of polymer,and is where a fibril originates or terminates with no clearcontinuation of the same fibril through the node. A fibril may have awidth of less than about 250 nm, or less than about 150 nm.

In forming the PLA polymers, lactides that are primarily eitherD-lactide or L-lactide are employed. In one or more embodiment, thelactide utilized in the polymerization process is a high purityL-lactide. It is to be appreciated that even though the lactide isprincipally either D-lactide or L-lactide, small amounts of the oppositeisomer, as well as small amounts of comonomers such as cyclic esters andcarbonates, may be incorporated into the polymer chain so long as theresultant polymer possesses the inherent viscosities and melt enthalpiesdescribed herein. Suitable comonomers include, but are not limited toglycolide, trimethylene carbonate, valerolacone, epsilon-caprolactone1,5-dioxepan-2-one, and 3,6-di-(n-butyl)-1,4-dioxane-2,5-dione. Theoptional comonomer(s) may be present in the PLA polymers in an amountfrom about 0.001 mol % to about 10 mol %, from about 0.01 mol % to about7 mol %, or from about 0.1 mol % to about 5 mol %.

The conversion of lactide to PLA is initiated by the ring openingpolymerization of the lactide. The ring opening may be catalyticallymediated by a metal compound, which may or may not also function as aninitiator. Additionally, an alcohol may also be added which, along withthe metal compound, forms a metal alkoxide in situ. Non-limitingexamples of suitable metal compounds include, but are not limited to,stannous 2-ethylhexoanate, Zn(II) oxide, and Zn(II) 2-ethylhexoanate. Inat least one embodiment, stannous 2-ethylhexoanate by itself or incombination with an alcohol, may be used as a catalyst. However, it isto be appreciated that any catalyst which is able to effect the ringopening polymerization of lactide at temperatures of 150° C. or lower issuitable for use.

The catalyst may be added in amounts ranging from about 1,500 mollactide/1 mole catalyst to about 200,000 mol lactide/1 mol catalyst,from about 3000 mol lactide/1 mol catalyst to about 100,000 mollactide/1 mol catalyst, or from about 7000 mol lactide/1 mol catalyst toabout 70,000 mol lactide/1 mol catalyst. In one or more embodiment,about 35,000 mol lactide/1 mole catalyst is added. Alcohol additionlevels range from zero to about 1,500 mol lactide/1 mole alcohol, fromabout 200,000 mol lactide/1 mol alcohol to about 3,000 mol lactide/1 molalcohol, or from about 70,000 mol lactide/1 mol alcohol to about 7,000mol lactide/1 mol alcohol. In at least one embodiment, 35,000 mollactide/1 mol alcohol may be added.

The polymerization reaction occurs at a temperature less than about 150°C., less than about 140° C., or less than about 110° C. The reaction maybe stopped at about 90 to 99 percent conversion of lactide to PLA, or atabout 95 to 99 percent conversion. The resultant PLA polymer is a solidmass, and may be in the form of a billet if the polymerization occurs inan appropriately sized and shaped container. If desired, the residuallactide may be removed from the PLA polymer by extraction using acetone.

Solution viscosity correlates with polymer chain length, and is commonlyused to monitor polymerization reactions and to characterize the finalpolymer. As such, the molecular weight of the PLA polymers formed fromthe process described above may be calculated from the solutionviscosity. The PLA polymers may have an inherent viscosity greater thanabout 3.8 dL/g, greater than about 4.5 dL/g, greater than about 5.0dL/g, greater than about 6.0 dL/g, or greater than about 7.0 dL/g. ThePLA polymers have a calculated molecular weight (Mw) greater than about190,000 g/mol, greater than about 250,000 g/mol, greater than about290,000 g/mol, greater than about 380,000 g/mol, or greater than about490,000 g/mol.

Bulk Polymerization Article Formation

The PLA polymer article formed from the bulk polymerization of PLApolymers may be cut, sawn, skived, milled, or otherwise partitioned ordivided into a PLA polymer preform that may be subsequently expandedinto a microporous article. It is to be appreciated that residualmonomer (unreacted monomers) may be removed (e.g., with acetone) beforeand/or after expansion. Preform shapes include sheets, films, rods,tubes, discs, and the like. To form a microporous article, the PLApolymer preform may be first heated to a temperature below the meltingpoint of the PLA polymer. In one or more embodiment, the preform isheated to a temperature of about 170° C. The PLA polymer preform maythen be expanded, which causes the crystalline PLA polymer to reorientand form beta phase crystals while simultaneously forming ahighly-fibrillated microporous structure. The presence of beta phasecrystals may be determined through wide angle X-ray diffraction (WAXD).The PLA polymer preform may be expanded at temperatures from at leastabout 1° above the glass transition temperature of the PLA polymer to atleast about 1° C. below the melt temperature of the PLA polymer, or fromabout 60° C. to about 185° C.

The WAXD pattern shown in FIG. 1 indicates no evidence of beta phasecrystals in the preform article. In contrast, the WAXD patterns shown inFIG. 2 of expanded PLLA are consistent with conversion of as polymerizedcrystalline phases into a beta crystalline phase. It is to be noted thatthe beta crystal is primarily located in the fibrils. A useful signalfor the presence of beta phase crystals is the observation by WAXD of(023)Beta peak depicted by reference numeral 20 and (003)Beta peakdepicted by reference numeral 10 as shown in FIG. 3. FIG. 3 is a 2-thetavs intensity plot of the WAXD patterns of FIG. 1 and FIG. 2 comparingthe integrated intensity of Example 2 with the meridonal intensity ofExample 3.

The degree of fibrillation, porosity, and beta-crystal content increaseswith increasing expansion, with expansions of 400%, 600%, and greaterbeing easily achieved. Expansion of the PLA polymer preform, eitheruniaxial or biaxial, may be conducted at rates up to 1,000%/second, upto 5,000%/second, up to 10,000%/second, or from about 1%/second to about5,000%/second, or from about 1% to 10,000%/second. The expansion ratiosmay be greater than 3:1, greater than 5:1, greater than 7:1, greaterthan 10:1, greater than 15:1, greater than 20:1, greater than 25:1,greater than 30:1, greater than 35:1, greater than 40:1, greater than45:1, or greater than 50:1. When the PLA polymer preform is expanded, itforms an expanded PLA polymer article that has a microstructure of nodesand fibrils, such as is shown at least in FIGS. 8, 9, and 10. FIGS. 8,9, and 10 depict exemplary nodes 30 and fibrils 40 in the expanded PLApolymer article.

In addition, the PLA polymer expanded article may be formed into adensified PLA material with a porosity less than about 10%, or less thanabout 5%. In one embodiment, the PLA polymer article may be compressedat a temperature from at least about 1° above the glass transitiontemperature to at least about 1° C. below the melt temperature of thePLA polymer, or from about 60° C. to about 185° C. The initial expandedform is largely opaque. The largely opaque sample may then be compressedto form a sample that is largely translucent and dense. It is to beunderstood to one of skill in the art that various times, temperatures,and pressure may be utilized to achieve a densified article.

PLA Powder Article Formation

In another embodiment, a microporous PLA polymer article is formed fromPLA powder. In this embodiment, the PLA polymers have an inherentviscosity of greater than about 3.8 dL/g and a calculated molecularweight greater than about 190,000 g/mol. The PLA polymers can be formedinto microporous articles at temperatures below the melting point of thePLA polymer.

The PLA polymer powder may be formed by the process set forth above,with the exception that the polymerization process is stopped at about40% completion. At lactide to PLA polymer conversion, the reaction massis a slush of liquid lactide and crystalline PLA polymer particles. Thisreaction mixture is permitted to cool and solidify. Once cooled, thesolidified mass of PLA polymer is broken into small pieces. In oneembodiment, the mass of PLA polymers is broken by hand. The lactide isextracted from the broken pieces with acetone in an appropriateextractor (e.g., a Soxhlet extractor), which leaves behind PLA polymerpowder. Agglomerations of the PLA polymer powder may be broken andsifted using an appropriately sized sizing screen.

Alternatively, the PLA polymer powder may be formed throughprecipitation in which poly-L-lactide is added to anhydrous o-xylene,heated, and then cooled to room temperature. A PLA precipitate is thenfiltered from the o-xylene. Hexanes may be added to the PLA precipitateto form a hexane/PLA slurry. A lubricant (e.g., a light mineral oil) maybe added to the slurry. A free-flowing PLA polymer powder is obtainedafter the hexanes have been evaporated.

As with the bulk polymerization described above, solution viscosity maybe used to calculate the molecular weight of the PLA polymers in apowder form. The PLA polymers may have an inherent viscosity greaterthan about 3.8 dL/g, greater than about 4.5 dL/g, greater than about 5.0dL/g, greater than about 6.0 dL/g, or greater than about 7.0 dL/g. ThePLA polymers have a calculated molecular weight (Mw) greater than about190,000 g/mol, greater than about 250,000 g/mol, greater than about290,000 g/mol, greater than about 380,000 g/mol, or greater than about490,000 g/mol.

Paste Processing PLA Polymer Powder

In one embodiment, the PLA polymer powder may be formed into amicroporous article through paste processing the PLA polymer powder. Informing a porous article from a PLA polymer powder, the PLA polymerpowder is first mixed with a lubricant, such as a light mineral oil.Other suitable lubricants include aliphatic hydrocarbons, aromatichydrocarbons, and the like, and are selected according to flammability,evaporation rate, and economic considerations. It is to be appreciatedthat the term “lubricant”, as used herein, is meant to describe aprocessing aid that includes (or consists of) an incompressible fluidthat is not a solvent for the polymer at the process conditions. Thefluid-polymer surface interactions are such that it is possible tocreate a homogenous mixture. It is also to be noted that that choice oflubricant is not particularly limiting and the selection of lubricant islargely a matter of safety and convenience. It is to be appreciated thatany of the lubricants described herein may be utilized as the lubricantso long as the fluid-polymer surface interactions are such that it ispossible to create a homogenous mixture. The lubricant may be added tothe PLA polymer powder in a ratio from about 1 ml/100 g to about 100ml/100 g or from about 10 ml/100 g to about 70 ml/100 g.

In at least one embodiment, the PLA polymer powder and lubricant aremixed so as to uniformly or substantially uniformly distribute thelubricant in the mixture. It is to be appreciated that various times andmixing methods may be used to distribute the PLA polymer powder in themixture. Once blended, PLA polymer powder/lubricant mixture is in apaste-like state. The PLA polymer powder can be formed into solid shapes(e.g. fibers, tubes, tapes, sheets, three dimensional self-supportingstructures, etc.) without exceeding the melt temperature of the PLApolymer. In one or more exemplary embodiment, the lubricated PLA polymerpowder is heated to a point below the melting temperature of the PLApolymer powder and sufficient pressure and shear are applied to forminter-particle connections and create a solid form. The lubricated PLApolymer powder can be formed into solid shapes such as fibers, tubes,tapes, sheets, three dimensional self-supporting structures, etc.without exceeding the melt temperature of the polymer. Non-limitingexamples of methods of applying pressure and shear include ram extrusion(e.g., typically called paste extrusion or paste processing whenlubricant is present) and calendering.

In one embodiment, the lubricated PLA polymer powder is ram extruded toproduce a cohesive tape. As used herein, the term “cohesive tape” ismeant to describe a tape that is sufficiently strong for furtherprocessing. The ram extrusion occurs below the melting temperature ofthe PLA polymer. In at least one alternate embodiment, the lubricatedPLA polymer powder may be calendered at a temperature below the meltingtemperature of the PLA polymer to produce a cohesive tape. Thecalendering occurs at temperatures from at least about 1° above theglass transition temperature to at least about 1° C. below the melttemperature of the PLA polymer, or from about 60° C. to about 185° C.The tape formed has an indeterminate length and a thickness less thanabout 1 mm. Tapes may be formed that have a thickness from about 0.01 mmto about 1 mm from about 0.08 mm to about 0.5 mm, or from 0.05 mm to 0.2mm, or even thinner. In exemplary embodiments, the tape has a thicknessfrom about 0.05 mm to about 0.2 mm.

In a subsequent step, the lubricant may be removed from the tape. Ininstances where a mineral oil is used as the lubricant, the lubricantmay be removed by washing the tape in hexane or other suitable solvent.The wash solvent is chosen to have excellent solubility for lubricantand sufficient volatility to be removed below the melting point of thePLA polymer. If the lubricant is of sufficient volatility, the lubricantmay be removed without a washing step, or it may be removed by heatand/or vacuum. The tape is then optionally permitted to dry, typicallyby air drying. However, any conventional drying method may be used aslong as the temperature of heating the sample remains below the meltingpoint of the PLA polymer.

The tapes, once dried, may be continuously processed or they may be cutto suitable sizes for expansion. Expansion of the samples occurs attemperatures below the melt temperature of the PLA polymer and above theglass transition temperature (Tg) of the PLA polymer. Expansion occursbelow the melting point of the PLA polymer, such as, for example, about80° C. below the melting point of the PLA copolymer, about 70° C. belowthe melting point, about 60° C. below the melting point, about 50° C.below the melting point, about 40° C. below the melting point, about 30°C. below the melting point, about 25° C. below the melting point, about15° C. below the melting point, about 10° C. below the melting point,about 5° C. below the melting point or about 1° C. below the meltingpoint. The samples may be expanded in one or more directions to form aporous PLA membrane. Additionally, the expansion ratios may be greaterthan 3:1, greater than 5:1, greater than 7:1, greater than 10:1, greaterthan 15:1, greater than 20:1, greater than 25:1, greater than 30:1,greater than 35:1, greater than 40:1, greater than 45:1, or greater than50:1.

The porous microstructure of the expanded membrane is affected by thetemperature and rate at which it is expanded. The geometry of the nodesand fibrils can be controlled by the selection of PLA polymer, the rateof expansion, temperature of expansion, and/or ultimate expansion ratio.

The expanded PLA polymer articles made in accordance with the processesdescribed herein have a matrix tensile strength greater than or equal to110 MPa, greater than or equal to 150 MPa, or greater than or equal to200 MPa. Further, the PLA polymer articles have a matrix modulus greaterthan or equal to 3000 MPa, greater than or equal to 4000 MPa, or greaterthan or equal to 5000 MPa.

In addition, the expanded PLA polymer articles have a percent porositythat is greater than about 10%, greater than or equal to about 15%,greater than or equal to about 20%, greater than or equal to about 25%,greater than or equal to about 30%, greater than or equal to about 35%,greater than or equal to about 40%, greater than or equal to about 45%,greater than or equal to about 50%, greater than or equal to about 55%,greater than or equal to about 60%, greater than or equal to about 65%,greater than or equal to about 70%, greater than or equal to about 75%,greater than or equal to about 80%, greater than or equal to about 85%,or up to (and including) 90%. In exemplary embodiments, the PLA polymerarticles may have a percent porosity from about 25% to about 90%, fromabout 40% to about 90%, from about 50% to about 90%, or from about 60%to about 90%.

Dry Processing PLA Polymer Powder

In yet another embodiment, the PLA polymer powder is calendered toproduce a cohesive dense tape. The dry processing of the PLA polymerpowder forms beta crystals. The calendering occurs at temperatures fromat least about 1° above the glass transition temperature to at leastabout 1° C. below the melt temperature of the PLA polymer, or from about60° C. to about 185° C. The tape formed has an indeterminate length anda thickness less than about 1 mm. Tapes may be formed that have athickness from about 0.01 mm to about 1 mm from about 0.08 mm to about0.5 mm, or from 0.05 mm to 0.2 mm, or even thinner. In exemplaryembodiments, the tape has a thickness from about 0.05 mm to about 0.2mm. It is to be understood to one of skill in the art that varioustimes, temperatures, and pressures may be utilized to achieve adensified article.

The dense tape may be continuously processed or may be cut to suitablesizes for expansion. Expansion of the dry tape occurs at temperaturesbelow the melt temperature of the PLA polymer and above and above theglass transition temperature (Tg) of the PLA polymer. In at least oneembodiment, the expansion occurs about 80° C. below the melting point ofthe PLA copolymer, about 70° C. below the melting point, about 60° C.below the melting point, about 50° C. below the melting point, about 40°C. below the melting point, about 30° C. below the melting point, about25° C. below the melting point, about 15° C. below the melting point,about 10° C. below the melting point, about 5° C. below the meltingpoint or about 1° C. below the melting point. The expansion may be inone or more directions to form a dense PLA article. Additionally, theexpansion ratios may be greater than 3:1, greater than 5:1, greater than7:1, greater than 10:1, greater than 15:1, greater than 20:1, greaterthan 25:1, greater than 30:1, greater than 35:1, greater than 40:1,greater than 45:1, or greater than 50:1.

The incorporation of filler materials and/or coatings in or on the PLApolymer articles described herein is considered to be within the purviewof the invention. For instance, a filler material may be blended with aPLA polymer before calendaring or ram extruding (and optionallyexpansion), or may be positioned on the PLA polymer article and lockedin place with a suitable hydrogel. Non-limiting examples of suitablefiller materials include inorganic materials (e.g., silica) carbonblack, aerogels, metals, semi-metals, ceramics, carbon/metal particulateblends, activated carbon, hydrogel materials, bioactive substances,stiffening agents, and combinations thereof. Filler materials may beincorporated into the PLA polymer article in amounts from about 1.0% toabout 80%, or from about 20% to about 60%, or from about 1% to about 30%by weight of the PLA article. Alternatively, suitable non-reactivefiller materials may be incorporated into the PLA polymer articlesduring polymerization of the PLA polymer.

Various components can be coprocessed with or placed on and/or withinthe PLA articles. In particular, components (or chemical compositions)may be added to the PLA polymer either during or after polymer synthesisin such a manner that the added component(s) become intimately mixed inthe polymer, such as in a blend or as a covalently bonded component ofthe PLA polymer chain. The added components could alternatively, oradditionally, be placed outside the polymer on surfaces of the fibrilsof the expanded PLA polymer. Further, the components may be placedwithin void spaces (e.g., pores) or between the fibrils in the expandedPLA article. The components added to or within the PLA article may beabsorbable or non-absorbable. The added compositions can include usefulsubstances that are releasably contained therein.

The components may include viscous chemical compositions, such as, butnot limited to, a hydrogel material. Biologically active substances mayoptionally be combined with a hydrogel material or with any other addedchemical component. With hydrogel materials, for example, thebiologically active substances may be released directly from thehydrogel material or they may be released as the hydrogel material andthe underlying expanded material are absorbed by the body of an implantrecipient.

Suitable hydrogel materials include, but are not limited to, polyvinylalcohol, polyethylene glycol, polypropylene glycol, dextran, agarose,alginate, carboxymethylcellulose, hyaluronic acid, polyacrylamide,polyglycidol, poly(vinyl alcohol-co-ethylene),poly(ethyleneglycol-co-propyleneglycol), poly(vinyl acetate-co-vinylalcohol), poly(tetrafluoroethylene-co-vinyl alcohol),poly(acrylonitrile-co-acrylamide), poly(acrylonitrile-co-acrylicacid-acrylamidine), poly(acrylonitrile-co-acrylic acid-co-acrylamidine),polyacrylic acid, poly-lysine, polyethyleneimine, polyvinyl pyrrolidone,polyhydroxyethylmethacrylate, polysulfone, mercaptosilane, aminosilane,hydroxylsilane, polyallylamine, polyaminoethylmethacrylate,polyomithine, polyaminoacrylamide, polyacrolein, acryloxysuccinimide, ortheir copolymers, either alone or in combination. Suitable solvents fordissolving the hydrophilic polymers include, but are not limited to,water, alcohols, dioxane, dimethylformamide, tetrahydrofuran, andacetonitrile, etc.

Optionally, the compositions can be chemically altered after beingcombined with the expanded PLA polymer. These chemical alterations canbe chemically reactive groups that interact with polymeric constituentsof the expanded PLA polymer or with chemically reactive groups on thecompositions themselves. The chemical alterations to these compositionscan serve as attachment sites for chemically bonding yet other chemicalcompositions, such as biologically active substances. “Bioactivesubstances” include enzymes, organic catalysts, ribozymes,organometallics, proteins, glycoproteins, peptides, polyamino acids,antibodies, nucleic acids, steroidal molecules, antibiotics,antimycotics, cytokines, carbohydrates, oleophobics, lipids,extracellular matrix material and/or its individual components,pharmaceuticals, and therapeutics. One non-limiting example of achemically-based bioactive substance is dexamethasone. Cells, such as,mammalian cells, reptilian cells, amphibian cells, avian cells, insectcells, planktonic cells, cells from non-mammalian marine vertebrates andinvertebrates, plant cells, microbial cells, protists, geneticallyengineered cells, and organelles, such as mitochondria, are alsobioactive substances. In addition, non-cellular biological entities,such as viruses, virenos, and prions are considered bioactive substancesherein.

Besides the utilization of added components for chemically orbiologically active functions, the added components may also (oralternatively) serve a physical or mechanical function. For example, theadded component can act as a void filler to facilitate furthermodification either prior to or during implantation. Such use of acarvable implant material may be of particular benefit for use insurgery, especially in plastic and reconstructive surgery. The materialmay include a porous expanded structure having a coating of abiocompatible stiffening agent to render the porous construct adequatelyrigid for carving to better adapt the implant for its intended use. Thecoating may be applied in a manner that allows the porous construct tobecome impregnated by the stiffening agent. Stiffening agents as usedherein would be composed of one or more absorbable materials, includingsynthetic biodegradable polymers and biologically derived materialswhich, if degrading faster than the base structure, would allow delayedingrowth of tissue into the porous construct after the stiffening agentis degraded through absorption.

Suitable materials for a polymeric biodegradable support member include,but are not limited to, polyglycolide (PGA), copolymers of glycolide,glycolide/L-lactide copolymers (PGA/PLLA), lactide/trimethylenecarbonate copolymers (PLA/TMC), glycolide/trimethylene carbonatecopolymers (PGA/TMC), polylactides (PLA), stereo-copolymers of PLA,poly-L-lactide (PLLA), poly-DL-iactide (PDLLA), L-lactide/DL-lactidecopolymers, copolymers of PLA, lactide/tetramethylglycolide copolymers,lactide/.alpha.-valerolactone copolymers, lactide/.epsilon.-caprolactonecopolymers, hyaluronic acid and its derivatives, polydepsipeptides,PLA/polyethylene oxide copolymers, unsymmetrical 3,6-substitutedpoly-1,4-dioxane-2,5-diones, poly-.beta.-hydroxybutyrate (PHBA),poly-4-hydroxybutyrate (P4HB), P4HB/PHBA copolymers,PHBA/bhydroxyvalerate copolymers (PHBA/HVA), poly-p-dioxanone (PDS),poly-a-valerlactone, poly-e-caprolactone,methacrylate-N-vinyl-pyrrolidone copolymers, polyesteramides, polyestersof oxalic acid, polydihydropyranes, polyalkyl-2-cyanoacrylates,polyurethanes, polyvinylalcohol, polypeptides, poly-B-malic acid (PMLA),poly-B-alcanoic acids, polybutylene oxalate, polyethylene adipate,polyethylene carbonate, polybutylene carbonate, and other polyesterscontaining silyl ethers, acetals, or ketals, alginates, and blends orother combinations of the aforementioned polymers. In addition to theaforementioned aliphatic link polymers, other aliphatic polyesters mayalso be appropriate for producing aromatic/aliphatic polyestercopolymers. These include aliphatic polyesters selected from the groupof oxalates, malonates, succinates, glutarates, adipates, pimelates,suberates, azelates, sebacates, nonanedioates, glycolates, and mixturesthereof. These materials are of particular interest as biodegradablesupport membranes in applications requiring temporary support, such asduring tissue or organ regeneration.

Test Methods

It should be understood that although certain methods and equipment aredescribed below, other methods or equipment determined suitable by oneof ordinary skill in the art may be alternatively utilized. It is to beunderstood that the following examples were conducted on a lab scale butcould be readily adapted to a continuous or semi-continuous process.

Scanning Electron Microscopy (SEM)

SEM images were collected using an Hitachi SU8000 FE Ultra HighResolution Scanning Electron Microscope with Dual SE detectors.Cross-sectioned samples were prepared using a Cooled straight-razorblade method. Surface and cross-sectioned samples were mounted onto a 25mm diameter metal stub with a 25 mm carbon double sided adhesive. Themounted samples were sputter coated with platinum.

Powder Wide Angle X-Ray Diffraction (WAXD

Diffraction patterns from calendered PLA powder were collected using aBruker Discovery D-8 instrument. The X-Ray source was CuKα element witha wavelength of 0.1542 nm running at 40 kV/60 mA. The instrument wasconfigured in a Brentano-Bragg geometry. Diffraction intensity wasmeasured using a OD scintillation counter rotating at 0.02 degree2-theta increments for a one second duration. The range of 2-theta was10 degrees to 35 degrees. The instrument was calibrated using apolycrystalline silicon and an automated internal calibration algorithm.PLA polymer was placed on the Bruker Discovery D-8 stage and alignedwith the beam line.

2-Dimensional Wide Angle X-Ray Diffraction (WAXD)—Method 1

Diffraction patterns from PLA polymer and expanded films were collectedusing a Molecular Metrology instrument configured for 2-D WAXDobservations. The X-Ray source was a Rigaku MicroMax Sealed Micro SourceCuKα element with a wavelength of 0.1542 nm running at 45 kV/66 mA. Tocollect two-dimensional diffraction information at wide angles a 20cm×20 cm Fujifilm BAS SR2040 imaging plate was placed in the instrumentvacuum chamber perpendicular to the X-Ray beam line at a camera lengthof 146 mm. Camera length was calibrated by collecting a WAXD patternfrom a tricosane standard and calculating the camera length from the 110reflection at q of 15.197 nm⁻¹ or d-spacing 0.4134 nm. PLLA polymerbillets approximately 1.5 mm thick were placed on a motorized stage andaligned perpendicular to the beam line. The vacuum chamber was thensealed and evacuated to 500 mTorr below atmospheric pressure and thebeam shutter opened. Diffraction patterns were collected at ambienttemperature for a period of 0.5 to 1 hour, depending on the thicknessand scattering intensity of the film sample. The diffraction data werecollected from the Fujifilm BAS SR2040 image plates using a GeneralElectric Typhoon FLA7000 image plate reader. Diffraction pattern imageswere saved as grayscale TIFF files and subsequently analyzed using POLARanalysis software.

2-Dimensional Wide Angle X-Ray Diffraction (WAXD)—Method 2

Diffraction patterns from PLA powder and films were collected using aSaxLab Ganesha instrument configured for 2-D WAXD observations. TheX-Ray source was a sealed CuKα element with a wavelength of 0.1542 nmrunning at 45 kV/66 mA. To collect two-dimensional diffractioninformation at wide angles a 0.3M Pilatus photon counting detector withpixel dimensions of 175 um×175 um was placed in the instrument vacuumchamber perpendicular to the X-Ray beam line at a camera length of 101mm. Camera length was calibrated by collecting a WAXD pattern from asilver behanate standard. Powder and film samples approximately 0.5 mmthick were placed on a motorized stage and aligned perpendicular to thebeam line. The vacuum chamber was then sealed and evacuated to 500 mTorrbelow atmospheric pressure and the beam shutter opened. Diffractionpatterns were collected at ambient temperature for a period of 10minutes. Diffraction pattern images were saved as grayscale TIFF filesand subsequently analyzed using POLAR analysis software.

Gurley Flow

The Gurley air flow test measures the time in seconds for 100 cm³ of airto flow through a 6.45 cm² aperture at 12.4 cm of water pressure. Anaperture of 0.645 cm² was actually used and the time observed divided bya factor of 10 to normalize observations for the aperture size. Theactual air volume was 300 cm³ and the time observed divided by anadditional factor of 3 to normalize observations for the volume size.Thus, the measured time was divided by a total factor of 30 to obtainthe Gurley flow. The samples were measured in a Gurley Densometer Model4110 Automatic Densometer equipped with a Gurley Model 4320 automateddigital timer. The reported result is the average of 3 measurements.

Differential Scanning Calorimetry

DSC data were collected using a TA Instruments Q2000 DSC between 0° C.and 250° C. using a heating rate of 10° C./min. Approximately 5 to 10 mgof the sample was placed into a standard Tzero pan and lid combinationavailable from TA Instruments. A linear integration method from 140° C.to 200° C. was used to integrate to obtain the first melt enthalpy data.

Solution Viscosity

A sample of the virgin polylactate polymer resin was dissolved intochloroform solvent. Using the results from proton NMR, the weight of thevirgin polymer resin added to the chloroform was adjusted to give asolution concentration of 0.1 g/dL of actual polymer. Solution wascharged to a Cannon #75 capillary L158 solution viscometer, which wasthen immersed in a 25.0 C water bath and equilibrated for 15 minutes.The formula used to calculate inherent viscosity was as follows:

$\eta_{inh} = \frac{\ln\frac{t}{t_{s}}}{C}$Where:  η_(inh) = inherent  viscosity  (dL/g)t = elution  time  of  the  solution  (s)t_(s) = elution  time  of  the  solvent  (s)${{C = {{polymer}\mspace{14mu}{concentration}\mspace{14mu}\left( {g\text{/}{dL}} \right)}};} = \frac{{\frac{1}{{polymer}\mspace{14mu}{fraction}} \cdot {wt}}\mspace{14mu}{virgin}\mspace{14mu}{polymer}\mspace{14mu}{resin}\mspace{14mu}(g)}{{Volume}\mspace{14mu}({dL})}$Solution-Viscosity Molecular Weight

A relationship between solution viscosity and molecular weight of PLLAin chloroform at 25° C. has been proposed as follows (D. W. Grijpma, J.P. Pennings, and A. J. Pennings, Colloid Polym Sci 272:1068-1081, 1994):[η]=5.45.10⁻⁴ ·M _(v) ^(0.73)  (Equation 1)

Where: [η]=intrinsic viscosity (dL/g)

M_(v)=Solution-viscosity molecular weight

Intrinsic viscosity may be calculated by measuring inherent viscosity atseveral concentrations, and in the region where the inherentviscosity-concentration relationship is linear, extrapolating a line tozero concentration (ASTM D2857-95). For polymer-solvent systems such asPLLA in chloroform, the slopes of the inherent viscosity vs.concentration line change very little for samples described herein. Theapproximate intrinsic viscosity was estimated from asingle-concentration measurement according to the following formula (F.W. Billmeyer, J Polymer Science 1949 4(1):83-86):

$\begin{matrix}{{{\lbrack\eta\rbrack = {\frac{4}{c}\left( {\eta_{r}^{1/4} - 1} \right)}}{{Where}{\text{:}\mspace{14mu}\lbrack\eta\rbrack}} = {{intrinsic}\mspace{14mu}{viscosity}\mspace{14mu}\left( {{dL}\text{/}g} \right)}}{\eta_{r} = {{{relative}\mspace{14mu}{viscosity}} \cong \frac{t}{t_{s}}}}{{c = {{polymer}\mspace{14mu}{concentration}\mspace{14mu}\left( {g\text{/}{dL}} \right)}};}} & \left( {{Equation}\mspace{14mu} 2} \right)\end{matrix}$

Substituting Equation 2 into Equation 1:

$\begin{matrix}{{\frac{4}{c}\left( {\eta_{r}^{1/4} - 1} \right)} = {5.45 \cdot 10^{- 4} \cdot M_{v}^{0.73}}} & \left( {{Equation}\mspace{14mu} 3} \right)\end{matrix}$Rearranging Equation 3 to solve directly for solution-viscositymolecular weight:

$\begin{matrix}{M_{v} = {e^{\frac{1}{0.73} \cdot {\ln{(\frac{\frac{4}{c}{({\eta_{r}^{1/4} - 1})}}{5.45 \cdot 10^{- 4}})}}}.}} & \left( {{Equation}\mspace{14mu} 4} \right)\end{matrix}$Thickness Measurement

Thickness was measured by placing the sample between the two plates of aMitutoyo Model ID-C112EX thickness gauge mounted on a Mitutoyo Model7004 cast base (Mitutoyo Corporation, Kawasaki, Japan). The average of 3measurements was reported.

Porosity Calculation

Porosity is reported as the volume fraction of void space measured inthe microporous PLA article. Density was used to calculate the porosityof expanded materials, using 1.23 g/cc as an approximation of the fulldensity of the PLA samples. The relative crystal content of the polymeraffects the density of the polymer. Uniaxially stretched samples weredie cut to form strips. Each sample was weighed using a Sartorius ModelMC 210 P balance, and then the thickness of the samples was taken usinga Mitutoyo thickness gauge (Mitutoyo Corporation, Kawasaki, Japan).Using this data, the bulk density of uniaxially expanded samples werecalculated with the following formula:

$\rho_{bulk} = \frac{m}{w \cdot l \cdot t}$

-   -   where: ρ_(bulk)=bulk density (g/cc)        -   m=mass (g)        -   w=width (cm)        -   l=length (cm)        -   t=thickness (cm).

Biaxially stretched samples were die cut as circles, and the bulkdensity calculated as follows:

$\rho_{bulk} = \frac{m}{\pi \cdot r^{2} \cdot t}$

-   -   where: ρ_(bulk)=bulk density (g/cc)        -   m=mass (g)        -   π=3.142        -   r=radius (cm)        -   t=thickness (cm)

Porosity is calculated as:

$P = {100 \cdot \left( {1 - \frac{\rho_{bulk}}{\rho}} \right)}$

-   -   where: P=% porosity        -   ρ_(bulk)=bulk density (g/cc)        -   ρ=polymer density, 1.23 g/cc.            Matrix Tensile Strength and Matrix Modulus

Tensile break load was measured using an INSTRON 5500R tensile testmachine equipped with flat-faced grips and a 900 N load cell. The gaugelength was 19 mm and the cross-head speed was 20.3 cm/min. Forlongitudinal MTS measurements, the larger dimension of the sample wasoriented in the calendering direction, which was designated the “machinedirection”. For the transverse MTS measurements, the larger dimension ofthe sample was oriented perpendicular to the calendering direction,which was designated the “transverse direction”.

The sample from the density measurement was used for tensile testing.The sample dimensions were 50 mm in length, 5 mm in width, andapproximately 0.5 mm thick. The effective thickness is calculated fromthe mass, the area, and the density of the sample. Two samples were thentested individually on the tensile tester. The average of the twomaximum load (i.e., the peak force) measurements was reported. Thelongitudinal and transverse MTS were calculated using the followingequation:MTS=(maximum load/cross-section area)*(density of “Polymer”)/density ofthe sample),

wherein the density of PLA is taken to be 1.23 g/mL.

Matrix modulus is calculated using the following equation:MatrixModulus=(small strain slope of load-displacementcurve/cross-section area)*(density of “Polymer”)/density of the sample.Proton Nuclear Magnetic Resonance (NMR)

A sample for ¹H Solution NMR collection was prepared by dissolvingapproximately 2 mg of polymer in approximately 2 mL CDCl₃. A BrukerBioSpin Avance II 300 MHz system was used to collect ¹H NMR data at300.13 MHz. A Bruker BioSpin 5 mm BBFO probe was installed in a standardbore 7.05T Bruker BioSpin ultra-shielded superconducting magnet.Temperature during NMR acquisition was 300K (26.9° C.). Software usedfor data acquisition and data processing was Topspin 1.3 or higher. Thedata was collected and processed using the conditions specified inTable 1. The spectra were referenced to the chloroform peak at 7.27 ppm.

The methine proton appearing as a quadruplet between 5.00 to 5.10 ppmwas assigned to lactate ester in the cyclic lactide form, and themethine proton appearing as a quadruplet between 5.12 to 5.24 ppm wasassigned to lactate ester in the polymer form. Relative amounts oflactate ester in the form of lactide or polymer were determined bycalculating the percent area of the quadruplet peaks with respect toeach other.

TABLE 1 NMR Acquisition Parameters ¹H NMR Frequency 300.13 MHzTransmitter offset 6.175 ppm Spectral Width 6188 Hz Pulse Length (30°)3.9 microseconds Acquired data points 198018 Acquisition Time 16 secondsRecycle delay 5 seconds Sample spinning speed 20 Hz Number of scans 128Total data points after zero fill 256k Line broadening 0.3 Hz

EXAMPLES Example 1

In a nitrogen purged glovebag, a 250 ml square glass bottle was filledto the top with L-lactide powder, then the bottle was closed with apolytetrafluoroethylene-lined polybutylene screw cap. The bottle wasplaced in a 130° C. oven and the L-lactide was melted. AdditionalL-lactide powder was added to the bottle, bringing the total chargeweight to 314.87 g (2.1846 mol) of L-lactide. The screw cap was looselyplaced upon the bottle, and the assembly placed in a vacuum chamber.Vacuum was drawn, then the chamber flooded with nitrogen. The bottle wasremoved from the chamber and the cap quickly tightened. The bottle wasplaced back in the 130° C. oven to continue the melting process.

When the L-lactide was completely melted, the cap was removed and 20.2μl (6.24×10⁻⁵ mol) of stannous 2-ethylhexoanate, and 6.54 μl (6.24×10⁻⁵mol) of 1,5-pentanediol were added to the bottle using apositive-displacement micro pipette. The bottle was vacuum/nitrogenpurged and swirled. The bottle was then placed in a 110° C. oven, andswirled occasionally over the next few hours. After 7 days in the oven,the bottle was removed. The polymer billet formed therein was freed bybreaking the glass bottle using a hammer.

Analysis by proton nuclear magnetic resonance (NMR) showed 91.00% oflactate ester as polymer. Differential Scanning Calorimetry (DSC)measured a peak melting temperature at 188.69° C. with a melt enthalpyof 68.19 J/g. The DSC thermogram is depicted in FIG. 4. The inherentviscosity was determined to be 9.64 dL/g, and the correspondingsolution-viscosity molecular weight was calculated to be 781,000 g/mol.

Example 2

The material of Example 1 was cut on a bandsaw into slices approximately1 mm thick, approximately 20 mm width, approximately 50 mm length. Awide angle x-ray diffraction (WAXD) pattern of the PLLA polymer is shownin FIG. 1. The circular WAXD pattern of FIG. 1 and the graphicaldepiction of FIG. 3 show an absence of beta crystals.

Example 3

The slices of Example 2 were drawn uniaxially in an MTS machine (810Model No. 318.10 commercially available from MTS Systems Corporation,Eden Prairie, Minn.) with a 2.5 kN MTS Force Transducer (Model No.661-18E-02, commercially available from MTS Systems Corporation, EdenPrairie, Minn.) equipped with a convection oven set to 170° C. Thesamples had a gauge length of 10 mm and were drawn uniaxially with acrosshead displacement rate of 0.1 mm/s. The total displacement was 40mm. A microporous structure was observed using scanning electronmicroscopy (SEM). An SEM of the surface of an interior view of theexpanded porous PLLA polymer taken at 40,000× magnification is shown inFIG. 8. The SEM shows the presence of nodes 30 and fibrils 40.Differential Scanning Calorimetry (DSC) measured the peak meltingtemperature of the polymer at 186.12° C. with a melt enthalpy of 54.56J/g. The DSC thermogram is depicted in FIG. 5. The multiple peaksdepicted in FIG. 5 indicate the presence of multiple crystal phases.

The WAXD pattern of FIG. 2 shows a single circular diffraction with ad-spacing that corresponds to the residual crystalline PLA monomer aswell as numerous discrete diffraction spots in the equatorial andmeridonal directions. This diffraction pattern and the associatedd-spacings and the graphical depiction of FIG. 3 indicate the presenceof beta crystals.

Example 4

The slices of Example 2 were drawn uniaxially in an MTS machine (810Model No. 318.10 commercially available from MTS Systems Corporation,Eden Prairie, Minn.) with a 2.5 kN MTS Force Transducer (Model No.661-18E-02, commercially available from MTS Systems Corporation, EdenPrairie, Minn.) equipped with a convection oven set to 170° C. Thesamples had a gauge length of 10 mm and were drawn uniaxially with acrosshead displacement rate of 1 mm/s. The total displacement was 60 mm.A microporous structure was observed using scanning electron microscopy(SEM). An SEM of the surface of an interior view of the expanded porousPLLA polymer taken at 20,000× magnification is shown in FIG. 9. The SEMshows the presence of nodes 30 and fibrils 40. Differential Scanningcalorimetry (DSC) measured a peak melting temperature of the polymer at185.69° C. with a melt enthalpy of 44.48 J/g. The DSC thermogram isdepicted in FIG. 6. The multiple peaks depicted in FIG. 6 indicate thepresence of multiple crystal phases.

Example 5

The slices of Example 2 were drawn uniaxially in an MTS machine (810Model No. 318.10 commercially available from MTS Systems Corporation,Eden Prairie, Minn.) with a 2.5 kN MTS Force Transducer (Model No.661-18E-02, commercially available from MTS Systems Corporation, EdenPrairie, Minn.) equipped with a convection oven set to 170° C. Thesamples had a gauge length of 10 mm and were drawn uniaxially with acrosshead displacement rate of 10 mm/s. The total displacement was 60mm. A microporous structure was observed using scanning electronmicroscopy (SEM). An SEM of the surface of an interior view of theexpanded porous PLLA polymer taken at 35,000× magnification is shown inFIG. 10. The SEM shows the presence of nodes 30 and fibrils 40.Differential Scanning calorimetry (DSC) measured a peak meltingtemperature of the polymer at 186.96° C. with a melt enthalpy of 40.70J/g. The DSC thermogram is depicted in FIG. 7. The multiple peaksdepicted in FIG. 7 indicate the presence of multiple crystal phases.

Example 6

The slices of Example 2 were drawn uniaxially in an MTS machine (810Model No. 318.10 commercially available from MTS Systems Corporation,Eden Prairie, Minn.) with a 2.5 kN MTS Force Transducer (Model No.661-18E-02, commercially available from MTS Systems Corporation, EdenPrairie, Minn.) equipped with a convection oven set to 170° C. Thesamples had a gauge length of 10 mm and were drawn uniaxially with acrosshead displacement rate of 100 mm/s. The total displacement was 60mm. A microporous structure was visually observed using scanningelectron microscopy (SEM).

Example 7

The slices of Example 2 were drawn uniaxially in an MTS machine (810Model No. 318.10 commercially available from MTS Systems Corporation,Eden Prairie, Minn.) with a 2.5 kN MTS Force Transducer (Model No.661-18E-02, commercially available from MTS Systems Corporation, EdenPrairie, Minn.) equipped with a convection oven set to 170° C. Thesamples had a gauge length of 10 mm and were drawn uniaxially with acrosshead displacement rate of 500 mm/s. The total displacement was 40mm. A microporous structure was visually observed using scanningelectron microscopy (SEM).

Example 8

The slices of Example 2 were drawn uniaxially in an MTS machine (810Model No. 318.10 commercially available from MTS Systems Corporation,Eden Prairie, Minn.) with a 2.5 kN MTS Force Transducer (Model No.661-18E-02, commercially available from MTS Systems Corporation, EdenPrairie, Minn.) equipped with a convection oven set to 180° C. Thesamples had a gauge length of 10 mm, were drawn uniaxially with acrosshead displacement rate of 10 mm/s. The total displacement was 60mm. A microporous structure was visually observed using scanningelectron microscopy (SEM).

Example 9

The slices of Example 2 were drawn uniaxially in an MTS machine (810Model No. 318.10 commercially available from MTS Systems Corporation,Eden Prairie, Minn.) with a 2.5 kN MTS Force Transducer (Model No.661-18E-02, commercially available from MTS Systems Corporation, EdenPrairie, Minn.) equipped with a convection oven set to 160° C. Thesamples had a gauge length of 10 mm and were drawn uniaxially with acrosshead displacement rate of 10 mm/s. The total displacement was 60mm. A microporous structure was visually observed using scanningelectron microscopy (SEM).

Table 1 sets forth the results of Examples 2, 3-9, and 11.

TABLE 1 Matrix MTS Modulus Porosity ID MPa MPa % Example 2 49.9 2898 0Example 3 175.7 5028 27.89 Example 4 217.0 4458 28.45 Example 5 172.53954 15.46 Example 6 84.9 4284 12.6 Example 7 159.6 4280 16.75 Example 8176.3 5002 20.47 Example 9 173.7 4947 14.23 Example 11 213.0 3919 21

Example 10

In order to get better release of the PLLA polymer from the glassreaction container, a 1 L cylindrical bottle was silanized by adding 1ml of octadecyltrichlorosilane and 20 ml of chloroform, closing with acap, shaking the bottle occasionally over several hours, and allowingthe mixture to stand overnight. The bottle was then emptied, rinsed 5times with 20 ml of chloroform for each rinse, then rinsed 2 times with100 ml of a 1:1 solution of methanol and water, then rinsed severaltimes with methanol and dried using nitrogen. The bottle was furtherdried by putting it into a 130° C. oven.

In a nitrogen purged glove box, 1000.2 g (6.934 mol) of L-lactide powderwas added to the silanized bottle, which was then closed with aPTFE-lined polybutylene cap. The bottle was placed in a 130° C. oven andthe L-lactide was melted. During the course of melting, degassing wasdone by drawing vacuum on the L-lactide to a level of about 3.0 Torr forone minute followed by pressurization with Nitrogen to 2 psig.

When the L-lactide was completely melted, the bottle was moved to anitrogen glove box, the cap was removed, and 211.9 μl of catalystsolution was added to the bottle using a positive-displacement micropipette. The catalyst solution consisted of 64.2 μl (1.98×10⁻⁴ mol) ofstannous 2-ethylhexoanate and 147.7 μl of anhydrous toluene. The bottlewas vacuum purged and swirled. The bottle was then placed in a 130° C.oven and swirled occasionally over a duration of 30 minutes. The bottlewas then placed in a 110° C. oven. After 3 days in the oven, the bottlewas removed. The polymer billet formed therein was freed by breaking theglass bottle.

Analysis by proton nuclear magnetic resonance (NMR) showed 93.6% oflactate ester as polymer. Differential Scanning Calorimetry (DSC)measured a peak melting temperature at 185.7° C. with a melt enthalpy of65.5 J/g. The inherent viscosity was determined to be 10.2 dL/g and thecorresponding solution-viscosity molecular weight was calculated to be853,000 g/mol.

Example 11

The material of Example 10 was cut on a lathe into a slice approximately1 mm thick. The slice was machined into a dogbone shape with a gaugelength of 6 mm. The dogbone sample was drawn uniaxially in a tensilemachine (Instron Model 5965 Norwood, Mass., 02062) equipped with aconvection oven set to 145° C. The crosshead displacement rate was 100mm/min and the total displacement was 62 mm. The porosity of theresultant expanded sample was calculated to be 21%.

Example 12

In order to get better release of the PLLA polymer from the glassreaction container, a 1 L cylindrical bottle was silanized by adding 1ml of octadecyltrichlorosilane and 20 ml of chloroform, closing with acap, shaking the bottle occasionally over several hours, and allowingthe mixture to stand overnight. The bottle was then emptied, rinsed 5times with 20 ml of chloroform for each rinse, and dried overnight in a110° C. oven.

In a nitrogen purged glovebag, 999.01 g (6.9313 mol) of L-lactide powderwas added to the silanized bottle, which was then closed with aPTFE-lined polybutylene cap. The bottle was placed in a 130° C. oven andthe L-lactide was melted.

When the L-lactide was completely melted, the cap was removed and 64.2μl (1.98×10⁻⁴ mol) of stannous 2-ethylhexoanate, and 20.7 μl (1.98×10⁻⁴mol) of 1,5-pentanediol were added to the bottle using apositive-displacement micro pipette. The bottle was vacuum/nitrogenpurged and swirled. The bottle was then placed in a 110° C. oven andswirled occasionally over the next few hours. After 16 days in the oven,the bottle was removed. The polymer billet formed therein was freed bybreaking the glass bottle using a hammer.

Analysis by proton nuclear magnetic resonance (NMR) showed 90.35% oflactate ester as polymer. Differential Scanning Calorimetry (DSC)measured a peak melting temperature at 178.24° C. with a melt enthalpyof 42.92 J/g. The inherent viscosity was determined to be 8.84 dL/g andthe corresponding solution-viscosity molecular weight was calculated tobe 683,000 g/mol.

Example 13

The material of Example 12 was cut using a bandsaw into slicesperpendicular to the cylinder axis to yield a disc approximately 5 mmthick and approximately 10 cm diameter. The disc was then milled on bothfaces to a consistent thickness.

Example 14

A disc from Example 13 having a thickness of 3.0 mm was restrained in anapparatus capable of stretching the disc in all directions in the planeof the disc at an equal rate within a temperature controlledenvironment. The sample temperature was approximately 168° C. and theradial displacement rate was 1.27 mm/s. The total radial displacementwas 134 mm. A microporous structure was observed using scanning electronmicroscopy (SEM). An SEM of the surface of an interior view of theexpanded porous PLLA polymer taken at 8,000× magnification is shown inFIG. 11. The porosity of the sample was calculated to be 62.2%.

Example 15

A disc from Example 13 having a thickness of 2.2 mm was restrained in anapparatus capable of stretching the disc in all directions in the planeof the disc at an equal rate within a temperature controlledenvironment. The sample temperature was approximately 168° C. and theradial displacement rate was 1.27 mm/s. The total radial displacementwas 221 mm. A microporous structure was observed using scanning electronmicroscopy (SEM). An SEM of the surface of an interior view of theexpanded porous PLLA polymer taken at 45,000× magnification is shown inFIG. 12. The porosity of the sample was calculated to be 48.2%.

Example 16

A disc from Example 13 having a thickness of approximately 2.0 mm wasrestrained in an apparatus capable of stretching the disc in alldirections in the plane of the disc at an equal rate within atemperature controlled environment. The platen temperature set-point wasset to 180° C. and the apparatus was permitted to equilibrate. Todirectly ascertain material temperature, a T-type thermocouple wasadhered to the top surface of the disc from Example 13 using Kaptontape. The disc was loaded into the apparatus chamber for a dwell time of10 minutes. After dwelling, the material temperature was observed to be168.4° C.

Example 17

Into a 2 oz square glass bottle 15.16 g (0.1052 mol) of L-lactide powderand 4.9 ul (1.5×10⁻⁵ mol) of stannous 2-ethylhexoanate was added. A foillined screw cap was loosely placed upon the bottle, and the assemblyplaced in a vacuum chamber. Vacuum was drawn, then the chamber floodedwith nitrogen. The bottle with cap was removed from the chamber and thecap quickly tightened. The bottle was placed in a 110° C. oven andoccasionally swirled over the next few hours during the melting process.After 6 days in the oven the bottle was removed.

Analysis by proton NMR showed 99.73% of lactate ester as polymer. DSCmeasured peak melting temperature at 190.47° C. with a melt enthalpy of84.34 J/g at a temperature ramp rate of 10° C./min. Inherent viscositywas 5.34 dL/g in chloroform at 25 C. at 0.1 g/dL polymer concentration.

Example 18

Into a 20 ml glass ampule 28.97 g (0.2010 mol) of L-lactide powder and9.3 μl (2.8×10⁻⁵ mol) of stannous 2-ethylhexoanate was added. A vacuumline was fitted to the stem of the ampule, vacuum was drawn, then theampule was flooded with nitrogen and the stem flame sealed. The ampulewas placed in a 110° C. oven and occasionally shaken during the meltingprocess. After 18 days in the oven the ampule was removed. Analysis byproton NMR showed 99.77% of lactate ester as polymer. DSC measured peakmelting temperature at 189.77° C. with a melt enthalpy of 61.84 J/g at atemperature ramp rate of 10° C./min. Inherent viscosity was 5.41 dL/g inchloroform at 25° C. at 0.1 g/dL polymer concentration.

Examples 19-25

Examples 19-25 were conducted using the synthesis method and ratios oflactide, stannous 2-ethylhexoanate, and 1,5-pentanediol described inExample 12, except that 250 ml square bottles were utilized. PLA wasprepared at various temperatures and various ratios of D and L isomercontent. Physical properties were characterized, and samples were pulledon the MTS tensile tester and examined for evidence of microporousstructure formation by scanning electron micrographs (SEMS). Results andparameters of Examples 3-9 and 19-25 are shown in Table 2. FIG. 13 is agraphical illustration of the data from Table 2 showing the formation ofa microporous structure as a function of melt enthalpy and inherentviscosity.

TABLE 2 PLA Physical Characteristics as Synthesized PLA Visc. DrawConditions Mole Synth. PLA Melt Inherent M.W. × Gauge Displacement TotalMicro Fract. L Temp. Melt Enthalpy Visc. 10³ g/ Length rate DisplacementOven Porosity Isomer C. Temp. C. J/g dL/g mole mm mm/sec mm Temp. CObserved Ex. 3 1 110 188.7 68.19 9.641 781 10 0.1 40 170 Yes Ex. 4 1 110188.7 68.19 9.641 781 10 1.0 60 170 Yes Ex. 5 1 110 188.7 68.19 9.641781 10 10.0 60 170 Yes Ex. 6 1 110 188.7 68.19 9.641 781 10 100.0 60 170Yes Ex. 7 1 110 188.7 68.19 9.641 781 10 500.0 40 170 Yes Ex. 8 1 110188.7 68.19 9.641 781 10 10.0 60 180 Yes Ex. 9 1 110 188.7 68.19 9.641781 10 100.0 60 160 Yes Ex. 19 1 140 178.1 46.30 7.210 497 10 10.0 40160 Yes 150 Yes Ex. 20 1 150 175.1 40.09 6.828 459 10 1.0 40 170 Yes 160Yes 150 Yes Ex. 21 1 110 191.6 73.25 4.456 248 5 0.5 20 170 No Ex. 22 1160 169.1 26.22 6.451 419 10 10.0 40 160 No 150 No 140 No 130 No 120 NoEx. 23 0.975 110 158.4 31.93 6.833 465 10 10.0 20 110 Yes Ex. 24 0.975125 153.7 27.20 4.978 291 10 10.0 40 130 No 120 No Ex. 25 0.950 125144.6 23.20 6.184 396 10 10.0 40 120 No

Example 26

The synthesis method described in Example 12 was used, except that a 250ml square bottle was utilized. 301.54 g (2.0940 mol) of L-lactide, 6.78μl (2.09×10⁻⁵ mol) of stannous 2-ethylhexoanate, and 2.19 μl (2.09×10⁻⁵mol) of 1,5-pentanediol were polymerized at 110° C. for 38 days.Analysis by proton nuclear magnetic resonance (NMR) showed 42.96% oflactate ester as polymer. The bottle was removed from the oven, and thepaste-like mixture of PLA/lactide was emptied from the bottle by meansof a spatula. The mixture was allowed to cool and solidify, was brokeninto small pieces, and the lactide extracted with acetone in a Soxhletextractor for 2 days. The recovered PLA powder was dried overnight in avacuum chamber.

Differential Scanning Calorimetry (DSC) measured peak meltingtemperature at 183.65° C. with a melt enthalpy of 84.12 J/g. Theinherent viscosity was determined to be 3.82 dL/g and the correspondingsolution-viscosity molecular weight was calculated to be 198,000 g/mol.

Example 27

The PLA powder of Example 26 was calendered at 110° C. to form a sheet.A scanning electron micrograph (SEM) of the cross-section of the sheettaken at 20,000× magnification is shown in FIG. 16. The properties ofthe PLA polymer sheet are set forth in Table 3.

TABLE 3 Thickness 0.8 mm Matrix Tensile Strength  35 MPa

Example 28

The synthesis method described in Example 12 was used, except that a 250ml square bottle was utilized. 329.58 g (2.2867 mol) of L-lactide, 3.70μl (1.14×10⁻⁵ mol) of stannous 2-ethylhexoanate, and 1.20 μl (1.14×10⁻⁵mol) of 1,5-pentanediol were polymerized at 110° C. for 52 days.Analysis by proton nuclear magnetic resonance (NMR) showed 40.37% oflactate ester as polymer. The bottle was removed from the oven, and thepaste-like mixture of PLA/lactide was emptied from the bottle by meansof a spatula. The mixture was allowed to cool and solidify, was brokeninto small pieces, and the lactide extracted with acetone in a Soxhletextractor for 1 day. The recovered PLA powder was dried overnight in avacuum chamber.

Differential Scanning Calorimetry (DSC) measured peak meltingtemperature at 194.55° C. with a melt enthalpy of 74.38 J/g. Theinherent viscosity was 5.37 dL/g and the correspondingsolution-viscosity molecular weight was calculated to be 325,000 g/mol.

Example 29

The PLA powder formed in Example 28 was lubricated with mineral oil at aratio of 40 ml/100 g of the PLA powder. The lubricated PLA powder wasfed into the nip of 2 counter rotating steel rolls 304.8 mm in diameterpre-heated to 110° C. with the gap between the rolls set at 0.127 mm.The speed was 0.914 meter/min. This process yielded a continuous,cohesive opaque tape. The tape was rinsed in hexane to remove themineral oil and allowed to air dry. Properties of the cohesive opaquetape are shown in Table 4. The porosity was measured from tape bulkdensity of 0.81 g/cc and a polymer density 1.23 g/cc. A scanningelectron micrograph (SEM) of the cross-section of the tape taken at15,000× magnification is shown in FIG. 15. A useful signal for thepresence of beta phase crystals is the observation by a powder XRD of(023)Beta peak depicted by reference numeral 20 and (003)Beta peakdepicted by reference numeral 10 as shown in FIG. 17. Properties of thecohesive opaque tape are shown in Table 4.

TABLE 4 Thickness  0.9 mm Matrix Tensile Strength 1.51 MPa Porosity 35%

Example 30

The PLLA powder formed in Example 26 was lubricated with mineral oil ata ratio of 40 ml/100 g of the PLLA powder. The lubricated PLA powder wasfed into the nip of 2 counter rotating steel rolls 304.8 mm in diameterpre-heated to 110° C. with the gap between the rolls set at 0.127 mm.The speed was 0.914 meter/min. This process yielded a continuous,cohesive opaque tape. A scanning electron micrograph (SEM) of thesurface of an interior view of the continuous, cohesive opaque tapetaken at 35,000× magnification is shown in FIG. 14.

Example 31

An opaque section of the expanded PLA article of Example 14 was fed intothe nip of 2 counter rotating steel rolls 304.8 mm in diameterpre-heated to 170° C. with the gap between the rolls set at 0.127 mm.The speed was 0.914 meter/min. The process yielded a sample that waslargely translucent. The gap between the rollers was reduced to 0.05 mm,and the largely translucent sample was passed through the counterrotating steel rolls. The sample that emerged was fully translucent,indicating that the sample was densified. The matrix tensile strength ofthe densified PLLA material as determined by ASTM D368 type V test wasdetermined to be 123.1 MPa. The result is an average of multiple tensilepulls from sibling samples cut from within the same densified article.

Example 32

A 24 mm PLA disc was die cut from the expanded porous PLLA described inExample 15. The thickness of the disc was 0.075 mm and the porosity wascalculated to be 50.6%. The disc was placed between 2 sheets of 0.125 mmthick polyimide film and compressed between the 152 mm×152 mm platens ofa heated press (Model No. 3312, Carver, Inc. Wabash, Ind.). The disc wasplaced in the press at 120° C. and 89 KN. The heat was immediatelyturned off and the PLLA disc was held under pressure as the press wasallowed to cool to 48° C. over a time period of 73 minutes. A 7 mm discwas cut from the compressed PLLA disc and the thickness and mass weremeasured. The porosity was calculated to be 6.6%. A scanning electronmicrograph (SEM) of the surface of the compressed PLLA polymer taken at30,000× magnification is shown in FIG. 28. Evidence of the fibrillatedstructure is visible in the image.

Example 33

The material of Example 1 was cut on a bandsaw into a sliceapproximately 1 mm thick, approximately 50 mm width, approximately 50 mmlength. The slice was drawn uniaxially in an MTS machine (810 Model No.318.10 commercially available from MTS Systems Corporation, EdenPrairie, Minn.) with a 2.5 kN MTS Force Transducer (Model No.661-18E-02, commercially available from MTS Systems Corporation, EdenPrairie, Minn.) equipped with a convection oven set to 170° C. Thesample had a gauge length of 20 mm and was drawn uniaxially with acrosshead displacement rate of 200 mm/s. The total displacement was 40mm. The sample was removed from the grips, rotated 90 degrees, trimmedto a width of 50 mm, and reloaded into the grips such that the seconddraw was perpendicular to the first draw. The sample had a gauge lengthof 20 mm and was drawn uniaxially a second time with a crossheaddisplacement rate of 20 mm/s. The total displacement was 20 mm. Theporosity was calculated to be 46.7%. The Gurley flow was determined tobe 0.468 seconds.

Example 34

Approximately 2 grams of PLLA from Example 12 was weighed into each offour 2-liter glass bottles, the total PLLA charge weight among the 4bottles being 8.39 g. The bottles were filled to the 1.8 L mark withanhydrous o-xylene, the headspace purged with nitrogen from a hose, anda PTFE-lined PBT cap quickly screwed into place. The four bottles wereplaced in a 140° C. oven, and occasionally swirled by hand. Afterapproximately 2.5 hours, the PLLA was solvated into the o-xylene, andthe oven temperature reduced to 70° C. for overnight. The bottles wereremoved from the oven the next morning, and allowed to cool and stand atroom temperature for another day. The PLLA precipitate was filtered fromthe o-xylene, placed into a 250 ml bottle, and the bottle filled withhexanes. The bottle was shaken, centrifuged, and decanted, with thiswashing process being repeated an additional 2 times. The PLLAprecipitate was transferred to a 500 ml wide-mouth glass jar by means ofa spatula, and the remaining PLLA precipitate on the sides of the 250 mlbottle was washed into the 500 ml jar using several 10 ml portions ofhexanes. A small amount of additional hexanes were added as needed toform a slurry by stirring with the spatula. 1.55 g of a light-grademineral oil lubricant was added to the PLLA/hexanes slurry, and slurrywas then stirred for several minutes. The hexanes were evaporated byimpinging an air stream into the 500 ml jar while continuously stirringwith the spatula. The slurry turned into crumbs, and the crumbs into afine free-flowing powder as the hexanes evaporated. The jar was placedin a vacuum chamber for several hours to remove the remaining hexanes.The jar was removed from the vacuum chamber and weighed, and had a netweight of 7.96 g. The mixture content of the resulting powder wascalculated to be 80.5% PLLA, 19.5% mineral oil by weight. A wide anglex-ray diffraction (WAXD) pattern of the PLLA powder is shown in FIG. 18.FIG. 19 is an intensity vs. 2-theta plot of the wide angle X-raydiffraction (WAXD) patterns of FIG. 18, which shows the absence of betacrystals. The inherent viscosity was determined to be 5.83 dL/g.

Example 35

A portion of the PLLA/mineral-oil powder of Example 34 was poured intothe nip of 2 counter rotating steel rolls 203.2 mm in diameter, having aroll speed of 0.914 meter/minute, a surface temperature of 125° C., andgapped at 0.025 mm. The resulting film was approximately 0.4 mmthickness. The film was immersed approximately 10 minutes in a hexanesbath to remove the mineral oil lubricant. This wash was repeated anadditional 2 times, with fresh hexanes being used each time. Theporosity of the film was calculated to be 26.4%. A scanning electronmicrograph (SEM) of the surface of an interior view of the calenderedPLLA film taken at 35,000× magnification is shown in FIG. 20. Theinherent viscosity was determined to be 6.04 d L/g.

Example 36

A portion of the calendered PLLA film from Example 35 was cut into astrip measuring 10.2 mm width, 0.390 mm thickness, and approximately 50mm in length. The strip was drawn in an Instron machine (Model No. 5965commercially available from Illinois Tool Works Inc., Norwood, Mass.),equipped with a convection oven set to 160° C. The sample gauge lengthwas set to 10 mm. After equilibrating 10 minutes in the oven, the samplewas then drawn uniaxially with a crosshead displacement rate of 1 mm/sand a total displacement of 100 mm. A microporous structure was observedusing scanning electron microscopy (SEM). The porosity was calculated tobe 49.34%. A scanning electron micrograph (SEM) of the surface of aninterior view of the expanded PLLA film taken at 45,000× magnificationis shown in FIG. 21. A wide angle x-ray diffraction (WAXD) pattern ofthe expanded PLLA film is shown in FIG. 22. FIG. 23 is an intensity vs.2-theta plot of the wide angle X-ray diffraction (WAXD) patterns of FIG.22, which shows the presence of beta crystals. The inherent viscositywas determined to be 5.98 dL/g.

Example 37

A portion of the calendered PLLA film from Example 35 was cut into instrip measuring 37.8 mm width, 0.434 mm thickness, and approximately 50mm in length. The strip was drawn in an Instron machine (Model No. 5965commercially available from Illinois Tool Works Inc., Norwood, Mass.),equipped with a convection oven set to 160° C. The sample gauge lengthwas set to 10 mm. After equilibrating 10 minutes in the oven, the samplewas then drawn uniaxially with a crosshead displacement rate of 1 mm/sand a total displacement of 60 mm. The sample was removed from thegrips, and the thickness measured to be 0.156 mm. The sample was trimmedto a length of 20.1 mm along the axis of draw, rotated 90 degrees andreloaded into the grips such that 20.1 mm was the new width. Afterequilibrating 10 minutes in the oven, the sample was then drawnuniaxially a second time, this instance being perpendicular to the firstdraw, with a crosshead displacement rate of 1 mm/s and a totaldisplacement of 40 mm. A microporous structure was observed usingscanning electron microscopy (SEM). The porosity was calculated to be47.31%. A scanning electron micrograph (SEM) of the surface of aninterior view of the PLLA film taken at 25,000× magnification is shownin FIG. 24. The inherent viscosity was determined to be 6.33 dL/g.

Example 38

A portion of the PLLA/mineral-oil powder of Example 34 was washed 3times with hexanes to remove the mineral oil. The mineral-oil free PLLApowder was poured into the nip of 2 counter rotating steel rolls 203.2mm in diameter, having a roll speed of 0.914 meter/minute, a surfacetemperature of 125° C., and gapped at 0.025 mm. The resulting film wasapproximately 0.7 mm in thickness. The porosity of the film wascalculated to be 5.66%.

Example 39

The calendered PLLA film from Example 38 was cut into in strip measuring10.8 mm width, 0.711 mm thickness, and approximately 50 mm in length.The strip was drawn in an Instron machine (Model No. 5965 commerciallyavailable from Illinois Tool Works Inc., Norwood, Mass.), equipped witha convection oven set to 160° C. The sample had a gauge length of 10 mm,and was drawn uniaxially with a crosshead displacement rate of 1 mm/s.The total displacement was 100 mm. A fibrillated structure. was observedusing scanning electron microscopy (SEM). The porosity was calculated tobe 6.98%. A scanning electron micrograph (SEM) of the surface of aninterior view the dense PLLA film taken at 25,000× magnification isshown in FIG. 25. A wide angle x-ray diffraction (WAXD) pattern of thePLLA film is shown in FIG. 26. FIG. 27 is an intensity vs. 2-theta plotof the wide angle X-ray diffraction (WAXD) patterns of FIG. 26, whichshows the presence of beta crystals.

Example 40

A powder consisting of PLLA and light-grade mineral oil lubricant wasprepared according to the method of Example 34, with a yield of 8.02 g.The mixture content of the resulting powder was calculated to be 80.4%PLLA and 19.6% mineral oil by weight.

Example 41

2.67 g of Syloid 620 silica powder (W. R. Grace & Co., Connecticut) wasadded to the PLLA/mineral oil powder of Example 40. This new mixture wasthoroughly mixed by tumbling the jar container for several hours. Themixture content of the resulting powder was calculated to be 60.3% PLLA,14.7% mineral oil and 25.0% silica by weight.

Example 42

A portion of the PLLA/mineral-oil/silica powder of Example 41 was pouredinto the nip of 2 counter rotating steel rolls 203.2 mm in diameter,having a roll speed of 0.914 meter/minute, a surface temperature of 125°C., and gapped at 0.025 mm. The first pass through the nip resulted inthe formation of large flakes. The flakes were collected and thematerial passed through the nip a second time, which resulted in theformation of a coherent film. The film was rotated 90 degrees and passedthrough the nip a third time, the orientation passing through the nipbeing perpendicular to the second pass. The film was rotated 90 degreesand passed through the nip a fourth time, the orientation passingthrough the nip being parallel to the second pass. The film was immersedapproximately 10 minutes in a hexanes bath to remove the mineral oillubricant. This wash was repeated an additional 2 times, with freshhexanes being used each time. After air drying, the film was placed in avacuum chamber to remove the remaining hexanes. The filled filmresulting from this process had a typical thickness of 0.75 mm, goodphysical integrity, and could be handled without cracking or visibleparticle shedding. The bulk density was calculated to be 0.767 g/ml.Using 1.23 g/ml as the basis density for PLLA, and 2.20 g/ml as thebasis density for silica, the average density of the PLLA/silica solidswas calculated to be 1.42 g/ml. The porosity of the PLLA/silica film wascalculated to be 46.0%. The matrix tensile strength of the PLLA/silicafilm was determined to be 15.8 MPa. A scanning electron micrograph (SEM)of the PLLA/silica film taken at 25,000× is shown in FIG. 29.

The invention of this application has been described above bothgenerically and with regard to specific embodiments. It will be apparentto those skilled in the art that various modifications and variationscan be made in the embodiments without departing from the scope of thedisclosure. Thus, it is intended that the embodiments cover themodifications and variations of this invention provided they come withinthe scope of the appended claims and their equivalents.

What is claimed is:
 1. A process for forming a porous articlecomprising: expanding a PLA polymer at a temperature above the glasstransition temperature of said PLA polymer and below a meltingtemperature of said PLA polymer to create a porous expanded PLA polymerarticle comprising a detectable beta crystal phase and nodes andfibrils.
 2. The process of claim 1, wherein said expanding occurs at atemperature from at least about 1° above the glass transitiontemperature of said PLA polymer to at least about 1° C. below the melttemperature of said PLA polymer.
 3. The process of claim 1, wherein saidexpanding occurs at a temperature from about 60° C. to about 185° C. toform said porous PLA article.
 4. The process of claim 1, furthercomprising compressing said porous PLA polymer article at a temperaturebelow said melting temperature of said PLA polymer to form a dense PLAarticle having a porosity less than about 10%.
 5. The process of claim1, further comprising adding at least one member selected from the groupconsisting of a filler material and a coating material to said porousPLA article.
 6. The process of claim 1, further comprising removingresidual monomer prior to said expanding below said melt temperature ofsaid PLA polymer.
 7. The process of claim 1, wherein said fibrilscomprise polymer chains and said polymer chains are oriented along afibril axis.
 8. The process of claim 1, wherein said expanded PLApolymer article is in the form of films, rods, tubes, or discs.
 9. Theprocess of claim 1, wherein said porous PLA polymer article has a matrixtensile strength greater than or equal to 110 MPa.
 10. The process ofclaim 1, wherein said porous PLA polymer article has a modulus greaterthan or equal to 3000 MPa.
 11. The process of claim 1, wherein saidporous PLA polymer article has a porosity greater than about 25%. 12.The process of claim 1, wherein said PLA polymer has an inherentviscosity greater than about 5 dL/g.
 13. The process of claim 12,wherein said PLA polymer has a molecular weight greater than about290,000 g/mol.
 14. The process of claim 1, wherein said PLA polymerarticle has a melt enthalpy greater than about 30 J/g.
 15. The processof claim 1, wherein said PLA polymer comprises at least one comonomer.16. The process of claim 1, wherein said PLA polymer comprises polyL-lactic acid (PLLA), poly d-lactic acid (PDLA), poly L-lactide, polyD-lactide, and combinations thereof.
 17. A process for forming amicroporous article comprising: lubricating a PLA polymer powder to forma lubricated PLA polymer; subjecting said lubricated PLA polymer topressure and to a temperature above the glass transition temperature ofsaid PLA polymer and below a melting temperature of said PLA polymer toform a preform; and expanding said preform at a temperature below themelt temperature of said PLA polymer to form a porous PLA article havinga structure of nodes and fibrils.
 18. The process of claim 17, whereinsaid subjecting comprises calendering said preform article below saidmelt temperature.
 19. The process of claim 17, wherein said calenderingoccurs at a temperature that is about 80° C. or less below said melttemperature.
 20. The process of claim 17, wherein said subjecting stepcomprises ram extruding said lubricated PLA polymer at a temperaturethat is below said melt temperature of said PLA polymer.
 21. The processof claim 20, wherein said ram extruding occurs at a temperature that isabout 80° C. or less below said melt temperature.
 22. The process ofclaim 17, further comprising removing said lubricant from said preformprior to said expanding.
 23. The process of claim 17, further comprisingcompressing said porous PLA article to form a dense article having aporosity of less than about 10%.
 24. The process of claim 17, furthercomprising adding at least one member selected from the group consistingof a filler material and a coating material to said microporous article.25. The process of claim 17, wherein said PLA polymer comprises polyL-lactic acid (PLLA), poly d-lactic acid (PDLA), poly L-lactide, polyD-lactide, and combinations thereof.
 26. The process of claim 17,wherein said PLA polymer comprises at least one comonomer.
 27. Theprocess of claim 17, wherein said porous PLA article has a matrixtensile strength greater than or equal to 110 MPa.
 28. The process ofclaim 17, wherein said porous PLA article has a matrix modulus greaterthan or equal to 3000 MPa.
 29. The process of claim 17, wherein saidporous PLA article has a porosity greater than about 25%.
 30. Theprocess of claim 17, wherein said PLA polymer has an inherent viscositygreater than about 3.8 dL/g.
 31. The process of claim 30, wherein saidPLA polymer has a molecular weight greater than about 190,000 g/mol. 32.The process of claim 17, wherein said PLA polymer has a melt enthalpygreater than about 30 J/g.
 33. The process of claim 17, wherein saidfibrils comprise polymer chains and said polymer chains are orientedalong a fibril axis.
 34. The process of claim 1, wherein said nodescomprise a volume of said expanded PLA polymer, and wherein said fibrilsoriginate and terminate from said nodes.